1. Field of the Invention
The present invention concerns a method for operation of an x-ray tube of the type having a number of emitters that generate respective electron beams, and an anode at which the electron beams strike on a surface to generate x-rays. The invention additionally concerns an x-ray tube with a number of emitters and a common anode.
2. Description of the Prior Art
An x-ray tube in its simplest form is composed of a cathode and an anode that are situated in a vacuum within a sealed glass body. In high-power tubes as they are used in computed tomography (CT) and angiography, the vacuum container is formed of metal which withstands significantly greater heat effects. In the course of time, tech improvements have also been made to the x-ray tubes but these changes have not changed the basic principle of the generation of x-rays.
To generate the x-rays, electrons are emitted from the cathode (the emitter) and are accelerated toward the anode by means of an applied high voltage. This electron beam penetrates into the anode material and is thereby braked (decelerated). In principle three different radiation types are generated by the braking of the individual electrons. One of these radiation types is the characteristic x-ray radiation that, depending on the anode material that is used (and therefore on the radiation structure), possesses a characteristic or, respectively, discrete spectrum and has its origin in a transition of electrons from high-energy shells of the atomic shell to low-energy shells. However, this characteristic x-ray radiation is not used (or is used only in small part) for image generation in an x-ray radioscopy, with the exception of mammography and crystal analysis.
The more important or greater part of the radiation types that is used is the x-ray bremsstrahlung. This arises due to the braking of electrons upon passing through the material of the anode. The wavelength of this radiation depends on the value of the acceleration (or braking), such that harder (i.e. higher energy) x-ray radiation is created at high acceleration voltage or anode voltage. The bremsstrahlung spectrum has a minimum wavelength at which the entire kinetic energy of the electron is emitted in a single photon. The third generated radiation type is the transition radiation or Lilienfeld radiation, but this cannot be employed in the medical use of x-ray tubes.
An x-ray tube with two emitters is known from DE 195 04 305 A1, for example. The one emitter generates a larger focal spot and the other emitter generates a smaller focal spot arranged within the larger focal spot on the anode, such that a resulting focal spot arises.
Application fields of x-ray tubes are, for example, in medicine in the radioscopy of bodies for analysis of illnesses or fractures or, respectively, in luggage inspection, or even for non-destructive materials testing (for example in the quality control of welding seams). The x-rays are thereby directed through the medium to be examined and captured by a photo plate or a similar image generating unit. The blackening of the photo plate is inversely proportional to the density of the medium being traversed. Fractures or material weakenings can be detected in a simple manner.
Particularly in the application of x-ray tubes in computed tomography, a high intensity or a variable setting of the intensity of the x-rays is frequently desired. However, for the most part this cannot be achieved in the x-ray tube due to structural and material-related limitations. In particular, the lifespan of the emitters is severely shortened given the generation of very high electron currents given lower high voltage values. Furthermore, an optimal focusing of the x-ray beam generated in the anode cannot occur at high electron currents since an expansion of the focal spot size on the anode ensues due to the repulsion of the electrons among one another due to space charge effects.